1. Field of the Invention
The invention relates to the field of design of new transmit beamforming methods for orthogonal frequency division multiplexing (OFDM) systems.
2. Description of the Prior Art
Frequency division multiplexing (FDM) is a technology that transmits multiple signals simultaneously over a single transmission path, such as a cable or wireless system. Each signal travels within its own unique frequency range (carrier), which is modulated by the data (text, voice, video, etc.). Orthogonal FDM's (OFDM) spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the “orthogonality” in this technique which prevents the demodulators from seeing frequencies other than their own. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion. This is useful because in a typical terrestrial broadcasting scenario there are multipath-channels (i.e. the transmitted signal arrives at the receiver using various paths of different length). Since multiple versions of the signal interfere with each other (inter symbol interference (ISI)) it becomes very hard to extract the original information. OFDM is sometimes called multi-carrier or discrete multi-tone modulation. It is the modulation technique used for digital TV in Europe, Japan and Australia.
The following are several published papers addressing transmit beamforming for quasi-static channels: K. Mukkavilli, A. Sabharwal, E. Erkip, and B. Aazhang, “On beamforming with finite rate feedback in multiple-antenna systems,” IEEE Trans. Inform. Theory, vol. 49, no. 10, pp. 2562-2579, October 2003; D. Love, R. Heath, and T. Strohmer, “Grassmannian beamforming for multiple-input multiple-output wireless systems,” IEEE Trans. Inform. Theory, vol. 49, no. 10, pp. 2735-2747, October 2003; S. Zhou, Z. Wang, and G. Giannakis, “Quantifying the power loss when transmit beamforming relies on finite rate feedback,” IEEE Trans. on Wireless Communications, to appear; and J. Choi and R. Heath, “Interpolation based transmit beamforming for MIMO-OFDM with limited feedback,” in IEEE International Conf. on Communications (ICC), vol. 1, pp. 20-24, June, 2004.
Among this prior art, the first three references addressed the transmit beamforming in quasi-static fading channels, and the last addressed a transmit beamforming scheme for quasi-static OFDM systems. In addition, J. Choi has exploited certain amount of inter-subcarrier correlations in the OFDM systems.
Multiple-input multiple-output (MIMO) systems offer much larger channel capacity over traditional single-input single-output (SISO) systems. Recently, many transmit beamforming methods have been developed to exploit the high capacity in the MIMO systems. The transmit beamforming schemes require certain amount of channel state information (channel state information) at the transmitter. Typically, the channel state information is conveyed from the receiver to the transmitter through a feedback link. It has been shown in the prior art that, even with limited feedback, a good beamforming scheme can provide significant amount of array processing gain. In a slow fading environment, the performance of the transmit beamforming methods is usually better than that of the open-loop methods (methods based on space-time coding). This is because extra channel information is utilized to fine tune the transmitted signal to fit the channel situation.
Orthogonal frequency division multiplexing (OFDM) is an efficient technique in broadband communications since it converts the original frequency selective channel into multiple parallel flat fading channels. Recently, the application of OFDM technology in MIMO systems (MIMO-OFDM) has attracted considerable attention from the research community. In MIMOOFDM, if certain amount of channel state information is available at the transmitter, transmit beamforming can also be employed to enhance the system performance. Ideally, a MIMO-OFDM beamformer can treat the MIMO channel as a collection of parallel narrow band channels, each operating on one particular subcarrier. Then transmit beamforming can be carried out on each subcarrier independently. However, the feedback link that conveys channel state information from receiver to transmitter is usually band limited.
For an OFDM system with a large number of subcarriers, independent beamforming on each subcarrier will incur a huge amount of channel feedback. Such a high feedback requirement is not sustainable by a practical wireless systems. Therefore, a MIMO-OFDM beamformer that can reduce the feedback requirement becomes highly desirable for OFDM wireless system design. To reduce the feedback requirement, a feasible approach is to exploit the strong time or frequency correlations among the different subcarriers. However, the traditional beamformer design has not addressed this important issue.
In one prior art beamforming scheme that exploits the frequency domain inter-subcarrier correlations to maintain the unit norm and resolve the phase ambiguity in the transmit weight, a method is employed using a spherical linear interpolator (SLI) for the beamformer design. Compared to the finite rate beamformers, the spherical linear interpolator beamformer significantly reduces the feedback requirement. In the meantime, it provides considerable performance improvement.